This invention relates, in general, to semiconductor devices, and more particularly to a method of electrolessly plating portions of semiconductor devices and the like.
It is desirable to plate portions of semiconductor devices and the like for many applications. This is especially true in the area of semiconductor package assembly where noble metals are often used to form ball bonds. However, the use of noble metals for ball bonds has serious detrimental effects. For example, it is often required that metal ball bonds be solder coated and noble metals such as gold have a very high dissolution rate in solder. This makes the noble metal extremely difficult to use.
If a metal such as nickel that has an extremely low dissolution rate in solder could be plated on a noble metal to obtain the most favorable characteristics of both metals, an excellent ball bond could be fabricated. However, noble metals generally are not chemically active enough to initiate electroless nickel deposition alone. Prior art methods of plating noble metals with nickel have included the use of a catalyst such as palladium chloride. However, the palladium chloride catalyst activates surfaces other than that of the ball bond and therefore, surfaces where plating is undesirable must be masked to prevent plating thereon. This makes it very difficult to plate small surfaces such as single ball bonds, a limited number of ball bonds on a single chip and other surfaces where it would be impractical, expensive and time consuming to employ a mask.
Flis et al. in an article entitled "Initiation of Electroless Nickel Plating on Copper, Palladium-Activated Copper, Gold, and Platinum", J. Electrochem.Soc.: Electrochemical Science and Technology, Vol. 131, No. 2, Page 254, February 1984, measured electrical activity at early states of nickel plating. They found that palladium-activated copper and gold were catalytically active and disclosed a plating solution comprising sodium hypophosphite, nickel sulphate, potassium sodium tartrate, ammonium hydroxide and sulfuric acid. However, the method disclosed by Flis et al. is not able to plate an appreciable amount of nickel on noble metals that were not palladium-activated.
Plating initiated by an outside electrical driving force is well known in the art. However, outside driving forces generally activate entire surfaces and therefore, plating of undesired surfaces is prevalent. As a result, masks, which again are often impractical in the plating of small surfaces, must be employed.
In view of the above, it would be highly desirable to have a method for electrolessly plating semiconductor devices and the like that requires no masks, relatively little time and is inexpensive.